A preferred field of application of the invention is in brushless dc motors and stepper motors as well as other permanent magnet motors that are configured as inner rotor motors. Inner rotor motors comprise a rotor arrangement that is mounted on a shaft and inserted coaxially into a stator arrangement. The stator body is generally made up of stamped metal laminations that carry windings. It comprises a back yoke ring on whose inner surface lie stator slots and stator poles that receive the stator winding.
In the prior art, electric machines are generally built with metal housings and mounting flanges that are disposed on one or both end faces of the housing. First, the stator is constructed as an assembly in that the stator body is built up to form, for example, a stamped lamination stack and is then wound. The rotor assembly is inserted into the stator, and the stator is fixed to the motor housing at the two mounting flanges using suitable means such as bonding, screwing, riveting, caulking, press fitting, welding etc. This kind of technique is also described in DE 695 02 496 T2 as discussed below.
In small electric motors, the available installation space is often limited so that the known methods of attaching the mounting flange on the motor housing either increase the size of the motor and, where installation space is limited, may no longer be an option, or the necessary process technology for assembly is complex and entails considerably more work and expense in production. What is more, conventional motor housings are not optimized for cooling the stator when in operation. The dissipation of heat to the environment simply takes place via the metal housing without any special arrangements being made for heat removal.
It is also known to totally dispense with an additional motor housing and to release the heat generated during operation directly from the stator surface into the environment. This is described, for example, in DE 10 2004 050 373 A1 as explained below.
DE 695 02 496 T2 describes a slotted stator for an electric machine that is made up of a plurality of stator laminations that are joined to form a lamination stack. This lamination stack forms a back yoke ring on whose inside surface lie stator slots and stator poles that receive the stator winding. The outside contour of the laminations takes the form of a regular polygon, preferably a rectangle whose edges are flattened to form a circular contour. The individual laminations are disposed at an angular offset with respect to one another such that, in projection, the flattened edges of the polygon complement one another to form a circle. In this way, the outside surface of the stator, and thus the exchange surface for dissipating heat to the environment, is made larger.
In DE 695 02 496 T1 the stator is disposed in a housing that has channels for conducting a cooling medium, such as air, between the inside surface of the housing and the outside surface of the stator, in order to cool the stator. This construction requires quite a lot of space and the special design of the housing including cooling channels is complex.
DE 10 2004 050 373 A1 describes a stator arrangement for an electric machine that is configured as an inner rotor motor, the stator having a back yoke ring and stator poles and stator slots formed on the inside surface of the back yoke ring. In order to insulate the stator poles with respect to the stator winding, an insulating coating is applied to the stator poles and stator slots. The insulating coating is injected onto the stator body such that the outside surface of the back yoke ring remains free of coating. A housing body is molded onto the stator, the housing body comprising two end-face housing sections. The bottom of the stator slots and the side surfaces of the stator poles are fully coated. The two end-face housing sections are aligned flush to the outside surface of the back yoke ring and directly adjoin this surface in order to form a sealed unit closed to the outside so that a separate housing need not be provided. The two end-face housing sections may each be connected to a flange. This goes to produce an electric machine having an extremely compact construction, which, however, in terms of cooling the stator is less favorable than the machine described initially.
It is an object of the invention to provide an electric machine that can be more easily manufactured at low cost and that has good properties in terms of heat dissipation of the stator.